In the casting arts, molten metal is poured into a mold through an opening in the top of the mold with a certain excess metal filling the channel to the mold cavity. When multiple parts are cast in the same casting vessel, the mold will include channels in the form of a sprue to each cavity for each part. Additionally, molten metal may solidify in vents and other areas and constitutes an imperfection of excess material on the surface of the casting. The excess metal, such as those referred to as risers or gates, may be partially removed after the cast part or casting has been sufficiently cooled however further machining, usually by grinding, is necessary to remove the remaining excess material from the casting. Note that the excess material may be regular or irregular in shape in each dimension and may be on a casting surface that is planar or arcuate or irregular itself.
Metal castings for the same part are inherently not dimensionally identical from casting to casting. These differences arise from mold variation, casting conditions, cooling rate, and other factors. Due to these variances, it is difficult to automate excess material removal using a device which follows a controlled predetermined path such as a robot. Specifically it is difficult to maintain tight tolerances between the casting and the ground surface of the excess, sometimes referred to as the witness, such that the proper amount of excess material is removed. For example a grinding tolerance may be flush to 0.060″, the robot is accurate to +/−0.002″, and the casting tolerance is +/−0.050″. With these parameters it will be impossible to grind to flush to 0.060″ without compensation.